Interpretive Summary: The carbon dioxide concentration (CO2) in the Earth’s atmosphere is increasing, and predictions of consequent global warming and changing precipitation patterns have been made. However, higher levels of CO2 are also known to directly stimulate the growth of plants, and because such larger plants would have more roots and residue, there is a potential for storage of more carbon in the soil. Therefore, as part of free-air CO2 enrichment (FACE) experiments, measurements of the effects of elevated CO2 on the growth of sorghum were made, and in addition, the amounts of soil carbon were determined that went into forms that decay both quickly and slowly. On average, about 53% of the soil organic carbon under elevated CO2 was in the slow pool and 47% in the fast pool, compared to 46% and 54%, respectively, for the ambient CO2 control plots. Moreover, most of change could be attributed to the new sorghum residue nearly doubling the amount carbon going to the slow pool compared to the fast pool. These results help to assess how much carbon can be sequestered in soils and thereby mitigate the rate of rise of atmospheric CO2 concentration, which will benefit many of earth’s ecosystems and most of mankind.

Technical Abstract:
Experimentation with dynamics of soil carbon pools as affected by elevated CO2 can better define the ability of terrestrial ecosystems to sequester global carbon. In the present study, 6 N HCl hydrolysis and stable-carbon isotopic analysis ('13C) were used to investigate the labile and recalcitrant soil carbon pools and the translocation among these pools of sorghum residues isotopically labeled in the 1998-1999 Arizona Maricopa Free Air CO2 Enrichment (FACE) experiment, in which elevated CO2 (FACE: 560 'mol mol-1) and ambient CO2 (Control: 360 'mol mol-1) interact with water-adequate (wet) and water-deficient (dry) treatments. We found that on average 53% of the final soil organic carbon (SOC) in the FACE plot was in the recalcitrant carbon pool and 47% in the labile pool, whereas in the Control plot 46% and 54% of carbon were in recalcitrant and labile pools, respectively, indicating that elevated CO2 transferred more SOC into the slow-decay carbon pool. Also, isotopic mixing models revealed that increased new sorghum residue input to the recalcitrant pool mainly accounts for this change, especially for the upper soil horizon (0-30 cm) where new carbon in recalcitrant soil pools of FACE wet and dry treatments was 1.7 and 2.8 times as large as that in respective Control recalcitrant pools. Mean residence time (MRT) of bulk soil carbon at the depth of 0-30 cm was significantly longer in FACE plot than Control plot by the averages of 18 yr and 20 yr under the dry and wet conditions, respectively. The MRT was positively correlated to the ratio of carbon content in the recalcitrant pool to total SOC and negatively correlated to the ratio of carbon content in the labile pool to total SOC. Influence of water alone on the bulk SOC or the labile and recalcitrant pools was not significant. However, water stress interacting with CO2 enhanced the shift of the carbon from the labile pool to recalcitrant pool. Our results imply that terrestrial agroecosystems may play a critical role in sequestrating atmospheric CO2 and mitigating harmful CO2 under future atmospheric conditions.